Lithium secondary batteries such as lithium ion batteries and lithium polymer batteries not only have high voltage and high capacity but also are light in weight compared to nickel cadmium batteries and nickel hydrogen batteries. In recent years, therefore, they are more widely used as main power sources for mobile communication equipment, portable electronic devices, electric bicycles, electric two-wheel vehicles, electric automobiles, etc. For example, generally, in current lithium ion batteries, as a positive electrode, lithium-containing transition metal compound oxide such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2) and lithium iron phosphate (LiFePO4) is used, and as a negative electrode, graphite, hard carbon or the like which is capable of absorbing and releasing lithium is used. In addition, as an electrolyte used in a lithium ion battery, a solution, in which an electrolytic salt is dissolved in a mixed organic solvent, is mainly used, wherein cyclic carbonate such as propylene carbonate (PC) and ethylene carbonate (EC), and chain carbonate such as dimethyl carbonate (DMC), diethyl carbonate (DEC) and methylethyl carbonate (MEC) are mixed in the mixed organic solvent, and wherein lithium tetrafluoroborate (LiBF4), lithium hexafluorophosphate (LiPF6), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium bis (trifluoromethanesulfonyl) imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonate (LiCF3SO3) or the like is used as the electrolytic salt.
As a major problem of current lithium ion batteries, there is given an uneven regional distribution of lithium resource. In view of this actual situation, the research of nonaqueous electrolyte secondary battery using sodium ion instead of lithium ion has been undertaken in recent years. Sodium is an element which is abundant in seawater and is the sixth most abundant element on earth, and therefore it is cheap and easy to obtain. That is, from the view point of the recent movement of not using rare earth elements, sodium is a very attractive element. In addition, as a power collector of negative electrode, a copper foil is used in a lithium ion battery; on the other hand, an aluminum foil which is cheap can be used in a sodium ion battery, which is an advantage over the lithium ion battery. Further, sodium is an alkali metal element to which lithium also belongs to, and thus it has similar characteristics to those of lithium. Accordingly, theory of sodium ion battery itself has been studied long before.
However, a sodium ion battery has a major problem. For example, a lithium ion battery causes intercalation phenomenon for charge and discharge, wherein lithium ion moves between graphite of a negative electrode active substance and lithium-containing transition metal compound oxide such as LiCoO2 of a positive electrode active substance, and moves between molecules of each material. Since graphite has a layered molecule structure, the structure of the graphite is rarely destroyed even if lithium ion moves in and out between layers. In addition, a lithium ion battery theoretically can absorb a capacity of 327 mAh/g of lithium ion. However, sodium ion has a large radius, and thus cannot enter between layers of molecules of graphite. Accordingly, a sodium ion battery does not show any capacity.
Patent Document 1 discloses an invention of a secondary battery using an alkali metal as negative electrode material. Specifically, it describes using a lithium metal as alkali metal. For example, if sodium metal (Na) as alkali metal is used as a negative electrode material, a high capacity can be theoretically achieved. However, when sodium metal (Na) is used as a negative electrode material, dendrites are precipitated on the negative electrode during charge, and after repeated charge and discharge they reach the positive electrode side, thus causing a phenomenon of internal short circuit. Furthermore, since precipitated dendrites have a high lability due to their large specific surface area, and interfacial membranes including solvent degradation products without electron conductivity are formed on the surface, the charge and discharge efficiency is lowered due to the high internal resistance of the battery. For these reasons, a sodium ion battery using sodium metal has disadvantages of low reliability and short cycle life.
From this background, a negative electrode material including other material than sodium metal and causing no internal short circuit has been desired in a sodium ion secondary battery. Patent Document 2 describes an invention relating to a sodium ion secondary battery using fibrous carbon material having a diameter of 0.1 μm to 1.0 μm as a negative electrode. When the fibrous carbon material described in Patent Document 2 is used as a negative electrode, the cycle life thereof is good. However, there exists a problem of low energy density.
Patent Document 3 concerning electrolyte describes that propylene carbonate or a mixed solvent of propylene carbonate and ethylene carbonate is preferably used as a solvent for electrolyte used for a hard carbon negative electrode in a sodium ion secondary battery. Non Patent Document 1 shows that a hard carbon electrode utilizing EC:DMC-based electrolyte commonly used in lithium ion batteries could not obtain good cycle characteristics in a sodium cell, and reports that better cycle characteristics are obtained from PC-based electrolyte than EC:DMC-based and PC:DMC-based electrolytes. In other words, Patent Document 3 and Non Patent Document 1 show that the cycle characteristics of negative electrode are largely dependent on electrolyte. Patent Document 4 discloses an example in which sodium ion is contained in nonaqueous electrolyte in a nonaqueous electrolyte secondary battery which includes a simple substance of Sn or Ge as a negative electrode.
However. Patent Document 3 and Non Patent Document 1 only studied a case of electrolyte used for a hard carbon negative electrode, and did not study a case of electrolyte used for an alloy-based negative electrode.
The negative electrode including a simple substance of Sn or Ge described in Patent Document 4 causes large volume expansion/contraction due to absorption/release of sodium at the time of charging/discharging. As a result, there is a problem that the electrode itself may fall apart, and thus the cycle life thereof is poor. In addition, Patent Document 4 did not study an electrolyte.